[Cffi-devel] What is the proper way to do GC in CFFI

[Stephan Frank]

Hi Joeish,

have a look at trivial-garbage (
http://common-lisp.net/project/trivial-garbage/). This project wraps the
finalizers of different Lisp implementations with a common interface.

So in your case it is best to wrap the pointer in a Lisp struct and attach
that the finalizer like this (not tested, so syntax errors may apply). You
can remove the type annotation of the struct slot if you don't want any
SBCL-specific code.

(defstruct (cvmatrix (:constructor %make-cvmatrix))
  (sap (mat) :type sb-sys:system-area-pointer :read-only t))

(defun make-cvmatrix ()
  (let* ((matrix (%make-cvmatrix))
         (sap (cvmatrix-sap matrix)))
    (tg:finalize matrix (lambda (x)
(del-mat sap)))
    matrix))


Now you create matrices with make-cvmatrix which returns you a cvmatrix
struct. Whenever this returned struct is garbage collected the finalizer is
called.
If you prefer classes to structs, of course a similar approach with
make-sintance and after methods can be easily implemented.

Regs,
Stephan



2014-04-10 5:41 GMT+02:00 Joeish W <joeish80829 at yahoo.com>:

> I have Lisp functions that wrap C wrappers for C++ functions that contain
> a new operator. The C wrappers have to stay the same I can't change those
> but I was advised to update my method of GC.
>
> Here are the C wrappers:
>
> Mat* cv_create_Mat() {
>     return new Mat();
> }
>
> void cv_delete_Mat(void* ptr) {
>     delete (~Mat*)ptr;
> }
>
> Here are the Lisp wrappers for them:
>
> (defcfun ("cv_create_Mat" mat) (:pointer mat)
>   "MAT constructor")
>
> (defcfun ("cv_delete_Mat" del-mat) :void
>   (ptr :pointer))
>
> In a loop, as below, I would normally after calling a MAT function, delete
> it when I'm done with it manually with DEL-MAT, but I heard there was a
> better way by actually entering in the DEL-MAT function into Lisps natural
> GC methods so it would know when it comes across a function with a new it
> should call delete to get rid of it when it know i'm done with it.. The
> thing is I could use help to get started.  Hopefully a concrete example of
> how to do this so I don't have to call DEL-MAT manually
>
> (dotimes (n 1000)
> (setf m (mat))
> (del-mat m))
>
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>
>
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